For transmitting antennas having a narrow tuned bandwidth, it is imperative to achieve an optimum performance level such that maximum radio frequency energy is radiated. This is especially true for applications in mobile radio, where the radio transmitters are typically power limited. One way to maximize the effective radiated power is to ensure that the impedance of the antenna is conjugately matched to the ounce of a transmit amplifier output stage by using a so-called pi-type impedance matching network having shunt capacitive circuit elements and one or more series inductive circuit elements.
Where the transmitter always operates on a fixed carrier frequency, or even over a narrow range of carrier frequencies, the proper antenna tuning can be performed in advance, by selecting the appropriate inductive and capacitive circuit elements to match the antenna impedance.
However, this process becomes more involved when the transmit frequency is not known in advance. In such applications, certain techniques have been developed to automatically tune the antenna to the desired transmit frequency.
One approach is to use a phase discriminator that compares the phase of a current or voltage at the driving point with the phase of a current or voltage in the main conductor of the antenna. The impedance value of an adjustable circuit element is then changed, depending upon the sign of the phase difference.
Other automatic antenna tuning systems attempt to minimize a standing wave ratio at the antenna by sensing a transmitted power and a reflected power. The ratio of these two power levels is then used to indicate a degree of match. Such a system is described in U.S. Pat. No. 5,225,847 issued to Roberts, D. A. and DeWitt, B. T. and assigned to Antenna Research Associates, Inc.
Unfortunately, even such automatic antenna tuning systems have their disadvantages. For example, in certain applications the antenna element or elements are electrically small compared to the operating wavelength of the system. In one such application, namely for mobile radio transmission in the high frequency (HF) radio band, at operating frequencies as low as 3 MegaHertz (MHz), the radio wave length is 100 meters or 328 feet. Obviously, if the radio is to be used in automobiles or as a portable device, practical antennas are necessarily small.
Furthermore, in vehicular applications in particular, the antenna may be regularly exposed to varying environmental conditions such as earth constants, and the elements such as mud, snow, ice, salt, corrosion, and the like, which from time to time may change the impedance which must be matched.
Thus, it has heretofore been thought that mobile radio transmitters must make use of expensive phase or standing ratio adjustment techniques to enable automatic tuning.